In recent years, the water absorbent resin has been extensively utilized as one of the component materials of such sanitary materials as disposable diapers and sanitary napkins with the object of absorbing body liquid. As the characteristic properties which the water absorbent-resin is expected to possess, the following three characteristic properties may be cited.
(1) Excellent absorption properties, namely a high capacity of water absorption, a high coefficient of water absorption under load, a high speed of absorption, and a high capacity of absorbing an aqueous liquid from a medium containing the aqueous liquid to be exhibited by the water absorbent resin when the resin is exposed to an aqueous liquid.
(2) Broad variety of objects at which the intended absorption is aimed. That is, the water absorbent resin manifests high affinity for not only water and urine but also such humors as blood and menses.
(3) Excellent handling properties manifested by the resin. That is, low hygroscopicity and high fluidity at elevated humidity, a small content of fine particles (less than 106 μm, for example), and low dusting characteristics.
Regarding the absorption properties, however, the individual properties mentioned in (1) above do not necessarily show a positive correlation. The solid state properties such as absorption velocity and gel strength are inevitably lowered in accordance as the speed of water absorption is increased, for example. The liquid permeability is increased but the absorption velocity is inevitably lowered in accordance as the average particle diameter of the water absorbent resin is increased.
As a means to modify the water absorption properties of the water absorbent resin in good balance, the technique of cross-linking the neighborhood of the surface of the water absorbent resin has been known. Various methods for embodying this technique have been proposed. For example, a method which cross-links 100 mass parts of a water absorbent resin with 0.2-20 mass parts of an inert solvent such as a polyhydric alcohol compound as a cross-linking agent and 0.005-5.0 mass parts of one compound selected from the group consisting of glycidyl ether compounds, haloepoxy compounds, aldehyde compounds, and isocyanate compounds in the presence of 0.01-1.3 mass parts of water based on 1 mass part of the water absorbent resin prior to a surface cross-linking treatment at a temperature in the range of 400-150° C. has been disclosed (U.S. Pat. No. 4,507,438 and U.S. Pat. No. 4,541,871). Since an increase in cross-linking density results in heightening speed of absorption but lowering water absorption power, this method is aimed at obtaining a water absorbent resin having excellent dispersibility in water and a high absorption velocity by causing a water absorbent resin swelled with a specific amount of water to be dispersed in an inert solvent and allowing a cross-linking agent to react on the resin.
Then, for the purpose of obtaining a water absorbent resin excelling in coefficient of water absorption under no load and in coefficient of water absorption under load as well, a method which produces an irregularly pulverized water absorbent resin by subjecting 100 mass parts of a water absorbent resin, 0.1-5 mass parts of a first cross-linking agent having an SP value of not less than 12.5 (cal/cm3) 0.005-0.5 mass parts of a second cross-linking agent having an SP value of not more than 12.5, and not more than 20 mass parts of water to a heat treatment at a temperature of not less than 160° C. has been disclosed (U.S. Pat. No. 5,422,405). As typical examples of the first cross-linking agent, ethylene glycol and propylene glycol are cited. As typical examples of the second cross-linking agent, diethylene glycol, 1,3-butane diol, polyethylene glycol diglycidyl ether, ethylene diamine, 2,4-trilene diisocyanate, and epichlorohydrin are cited. In a working example, the cross-linking agents are caused to react on 100 mass parts of the water absorbent resin in the presence of 3-5 mass parts of water. Incidentally, the average particle diameter of the water absorbent resin to be used is most preferably in the range of 300-600 μm.
A highly absorbent polymer which has an equilibrium swell rate of absorption of not less than 40 g/g of physiological saline water and a time of not more than 40 seconds for passage of 5 ml of physiological saline water as determined by a prescribed method has been also known (GB-B-2,267,094). The polymer is obtained by heightening the cross-linking density of the surface of a water-insoluble hydrophilic cross-linked polymer possessing a carboxyl group or a carboxylate group. In a working examples cited therein, a water-insoluble hydrophilic cross-linked polymer possessing a carboxyl group or a carboxylate group is made to add 2,500 ppm of a surface cross-linking agent under the condition of containing water at a concentration in the range of 20-35 mass % to effect addition to the cross-linking density of the surface of the particles of this cross-linked polymer.
Further, as a water absorbent resin suitable for sanitary materials having a high resin concentration, a water absorbent resin which has the thickness of the surface cross-linked layer thereof decreased to the order of submicrons and has the ratio, mass %, of the surface cross-linked layer to the whole water absorbent resin controlled in a specific range has been disclosed (JP-A-2001-192,464). Since an undue increase in the surface cross-linked layer results in obstructing the water absorption power of the water absorbent resin and inflicting a crack in the surface cross-linked layer during or after the course of swelling, the thickness of the surface cross-linked layer is required to be not less than 50 nm and the ratio of the surface cross-linked layer is restricted to a range of 0.3-3 mass %. The water absorbent resin of this quality is obtained preferably by using a polyhydric alcohol as a surface cross-linking agent and controlling the temperature of the water absorbent resin in a range of 5°-20° C. and/or fixing the temperature of the surface cross-linking agent-containing liquid in the range of 0°-20° C. In a working example, the surface cross-linking treatment is carried out in the presence of 2-4 mass parts of water based on 100 mass parts of the water absorbent resin which has not undergone the surface cross-linking treatment.
Meanwhile, a method which promotes uniform dispersion of a surface cross-linking agent on the surface of a water absorbent resin and proper permeation thereof in the neighborhood of the surface, prevents the adjacent particles from coalescing, and accomplishes uniform surface cross-linkage by adjusting the average particle diameter of the water absorbent resin in the range of 100-600 μm and the logarithmic standard deviation δζ below 0.35 has been disclosed (U.S. Pat. No. 4,973,632, U.S. Pat. No. 5,026,800, U.S. Pat. No. 5,244,735, and U.S. Pat. No. 6,087,002). The water absorbent resin which has not undergone the surface cross-linking treatment is obtained by using the aqueous solution of a water-soluble ethylenically unsaturated monomer having a viscosity of not less than 15 cps as determined preferably with a Brookfield rotational viscosimeter under the conditions of 25° C. and 0.6 rpm and using a sucrose fatty acid ester and/or a polyglycerin fatty acid ester as a dispersant, dispersing and suspending them in a hydrophobic organic solvent inert to polymerization, and setting the monomer polymerizing with a radical polymerization initiator. Then, the surface cross-linked water absorbent resin is obtained by mixing a water absorbent resin powder dried to a water content of less than 10% with 0.005-20 mass % of a surface cross-linking agent, 0.1-5 mass % of water, and 0.01-6 mass % of a hydrophilic organic solvent and subsequently subjecting the resultant mixture to a thermal reaction.
The surface cross-linked water absorbent resins which are obtained by the heretofore well known surface cross-linking techniques enumerated above indeed bring an effect of exalting the coefficient of water absorption under load and the absorption velocity described in (1) above. They, however, entail the problem that the coefficient of water absorption determined under load is vastly lowered. Further, they tend either to induce absolutely no improvement of the affinity for blood described in (2) and the hygroscopicity at elevated temperatures described in (3) above or to rather aggravate them.
While the affinity for blood and the hygroscopicity at elevated temperatures are thought to depend on the characteristic properties of the surface of the water absorbent resin, the reports published to date have paid no attention whatever to such surface properties and have been substantially wholly directed toward solely evaluating the absorption properties. They have made virtually no effort to express the homogeneity of heterogeneity of the cross-linkage resulting from the surface cross-linking treatment or the difference of the chemical properties of surface. The desirability of developing a technique for clarifying conveniently and distinctly the homogeneity and heterogeneity of the cross-linkage and the difference of chemical properties of surface has been finding growing recognition.
Further, in consequence of the recent years' advance of the absorbent products toward decreased thicknesses and of the development of processing techniques directed toward the decreased thicknesses, the desirability of developing a water absorbent resin which has a high coefficient of water absorption, a small average particle diameter, and a high absorption velocity, and also has a high total absorption capacity has been finding recognition. When the conventional methods are adopted for the production of a water absorbent resin having such a high coefficient of water absorption and such a small average particle diameter as mentioned above, the high absorption velocity of the water absorbent resin has prevented the aqueous solution of a surface cross-linking agent from being uniformly applied to the surface of the water absorbent resin particles and the surface cross-linking treatment from being carried out uniformly.
Thus, this invention has for an object thereof the provision of an excellent water absorbent material which retains a high coefficient of water absorption and a high absorption velocity and manifests a high total absorption capacity as well and a method for the production thereof.
It has another object of providing a water absorbent material which is formed of a surface cross-linked water absorbent resin possessing affinity for blood and menses and exhibiting an excellent handling property and a method for the production of such a surface cross-linked water absorbable resin as mentioned above.
It has still another object of providing a method for convenient and distinct evaluation of the surface properties (cross-linking property and chemical property) of the absorbent material.